Ch 1 · 1.1 The microscope in cell studies · AS & A Level Biology 9700
Topic 1 of 2 in Cell structure — AS & A Level Biology 9700 — 77 lessons.
Introduction
Every A Level Biology practical begins with a microscope. This lesson covers how to make a slide and draw what you see, the formula linking image size, actual size and magnification, calibrating an eyepiece graticule, and the resolution limits that explain why electron micrographs reveal detail no light microscope can show. (Syllabus section 1.1.)
Key ideas — Magnification and resolution
Magnification is the number of times larger an image is than the actual object. Resolution is the minimum distance between two points at which they can still be distinguished as two separate points, rather than one blur. Increasing magnification without sufficient resolution simply produces a larger, blurred image with no extra detail: resolution, not magnification, sets the limit on how much structure can ever be seen. Resolution is determined by the wavelength of the radiation used to form the image, because two points closer together than roughly half that wavelength cannot be resolved.
🧑🏫 Teacher's explanation
Imagine you are looking at a digital photo on your phone. Magnification is like 'pinching to zoom' on the screen; you make the pixels larger, but if the photo was low quality to begin with, it just looks blurry and pixelated. You aren't seeing new details, just bigger versions of the same blurry blobs.
📖 Definitions
🔍 Worked example — Easy
A student views a plant cell under a light microscope. The image of the cell is 4.0 mm long. The actual length of the cell is 80 μ m. Calculate the magnification.
🔍 Worked example — Standard
An electron micrograph shows a mitochondrion that is 25 μ m long in the image. The actual length of the mitochondrion is 2 μ m. Calculate the magnification.
🔍 Worked example — Exam-style
A student uses an eyepiece graticule and a stage micrometer to calibrate the microscope. The stage micrometer has 10 divisions spanning 0.2 mm. If the eyepiece graticule scale shows 50 divisions for the same distance, calculate the value of one eyepiece graticule division in μ m.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the magnification of the image. [2] (b) Explain why an electron microscope can resolve smaller structures than a light microscope. [2] [4 marks]
Key ideas — Calculating magnification and actual size
magnification = size of image/size of real object
🧑🏫 Teacher's explanation
Think of magnification like a zoom feature on your smartphone camera. When you zoom in, the object on your screen looks much larger than the actual object in real life. However, the object itself hasn't actually grown; you are just creating a larger representation of it.
📖 Definitions
🔍 Worked example — Easy
A student measures a cell in a photomicrograph. The image of the cell is 45 mm long. The actual size of the cell is 0.015 mm. Calculate the magnification.
🔍 Worked example — Standard
An electron micrograph shows a mitochondrion that is 20mum long. The image of the mitochondrion is 50 mm long. Calculate the magnification.
🔍 Worked example — Exam-style
A bacterium has an actual length of 2mum. It is viewed under a light microscope with a total magnification of × 400. Calculate the length of the image produced in mm.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the magnification of the image. [2] (b) Explain why an electron microscope has a higher resolution than a light microscope. [2] [4 marks]
Key ideas — The eyepiece graticule and stage micrometer
An eyepiece graticule is a small scale of arbitrary units fitted inside the eyepiece; its divisions do not correspond to a fixed real distance until calibrated. A stage micrometer is a microscope slide with an accurately ruled scale of known distance, typically divided into 0.01 mm units, placed on the stage.
🧑🏫 Teacher's explanation
Imagine you are looking at a ruler through a piece of tinted glass that has its own markings printed on it. The markings on the glass (the eyepiece graticule) don't change size relative to your vision, but the ruler underneath (the stage micrometer) stays the same. If you move the glass closer or further, or change the zoom on your lens, the markings on the glass and the ruler won't line up the same way anymore.
📖 Definitions
🔍 Worked example — Easy
A student aligns an eyepiece graticule with a stage micrometer. 10 divisions on the eyepiece graticule coincide exactly with 8 divisions on the stage micrometer. Each division on the stage micrometer is 0.01 mm. Calculate the distance represented by one eyepiece division.
🔍 Worked example — Standard
Using the calibration from the previous example (1 eyepiece unit = 0.008 mm), a specimen is measured to be 15 eyepiece units long. Calculate the actual size of the specimen in micrometres (mum).
🔍 Worked example — Exam-style
An image of a chloroplast is 45 mum long. The eyepiece graticule was calibrated such that 1 unit = 2 mum. The image was taken using a microscope with a total magnification of × 400. Calculate the actual size of the chloroplast.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student aligns an eyepiece graticule with a stage micrometer. 20 divisions of the eyepiece graticule coincide with 16 divisions of the stage micrometer. Each division on the stage micrometer represents 0.01 mm. Calculate the distance represented by one division of the eyepiece graticule in mum. [2] (b) The specimen is measured to be 25 eyepiece units long. Calculate the actual size of the specimen in mum. [2] [4 marks]
Key ideas — Converting between units of length
Cell biology spans several orders of magnitude, so conversions between millimetres (mm), micrometres (μm) and nanometres (nm) must be automatic: 1 mm = 1000 μm, and 1 μm = 1000 nm, so 1 mm = 1 000 000 nm. Most eukaryotic cells are 10–100 μm in diameter; organelles such as mitochondria are 1–10 μm; ribosomes and membranes are measured in nanometres.
🧑🏫 Teacher's explanation
In biology, we deal with things that are incredibly small. If you tried to measure a single bacterium using a standard ruler marked in centimetres, the ruler would be useless. It's like trying to measure the thickness of a single human hair using a map of an entire city; the scale is just too large. To be precise, we use a 'nested' system of units where each step is exactly 1000 times smaller than the last.
📖 Definitions
🔍 Worked example — Easy
A student views a plant cell under a light microscope. The image of the cell is 45 mm long. Calculate the actual length of the cell in micrometres (mum) if the magnification is × 450.
🔍 Worked example — Standard
A mitochondrion has an actual diameter of $2\text{ $\mu\text{m}. It is photographed for a textbook. If the photograph shows the mitochondrion as 20 mm wide, calculate the magnification used.
🔍 Worked example — Exam-style
An electron micrograph shows a ribosome with a width of 50 nm. The image width is 25 mm. Calculate the magnification.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Light microscopes versus electron microscopes
The light microscope uses visible light (wavelength roughly 400–700 nm) and has a maximum resolution of about 200 nm, limiting useful magnification to roughly ×1500. The electron microscope uses a beam of electrons, whose much shorter wavelength gives a maximum resolution of about 0.5 nm, allowing magnifications beyond ×500 000.
🧑🏫 Teacher's explanation
Imagine you are looking at a distant mountain range. Magnification is like using a telescope to make the mountain look larger on your retina; it makes things bigger, but it doesn't necessarily make them clearer. Resolution, however, is like the quality of the lens; it determines whether you see a blurry green blob or can actually distinguish individual trees on the mountainside.
📖 Definitions
🔍 Worked example — Easy
A student views a mitochondrion under a light microscope. The image of the mitochondrion measures 40 mm in length. The actual length of the mitochondrion is 5 μ m. Calculate the magnification.
🔍 Worked example — Standard
An electron micrograph shows a section of chloroplast with a width of 25 mm. The actual width of the chloroplast is 25 μ m. Calculate the magnification.
🔍 Worked example — Exam-style
A researcher uses a Scanning Electron Microscope (SEM) to view a pollen grain. The image produced is 120 mm wide. The actual diameter of the pollen grain is 50 μ m. Calculate the magnification.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the magnification of the image. [1] (b) Explain why the mitochondrion cannot be viewed using a light microscope if the goal is to see the individual cristae. [2] [3 marks]
Key ideas — Making a temporary preparation
A temporary preparation (a "wet mount") lets you look at fresh cells within minutes:
🧑🏫 Teacher's explanation
Imagine you are trying to look at a very thin, clear piece of tissue paper through a flashlight. If the paper is too thick, the light won't pass through; if the paper is perfectly clear, you won't see any detail. To see the details, you need to add a tiny bit of colored ink (a stain) to make the structures stand out. This is exactly why we use stains and thin sections in microscopy.
📖 Definitions
🔍 Worked example — Easy
A student observes a plant cell under a light microscope. The image of the cell measures 40 μ m in length. The magnification used was × 400. Calculate the actual size of the cell in micrometres (\mu m).
🔍 Worked example — Standard
An electron micrograph shows a mitochondrion with an image length of 15 mm. The magnification is × 20,000. Calculate the actual length of the mitochondrion in micrometres (\mu m$).
🔍 Worked example — Exam-style
A stage micrometer is 1 mm long and is divided into 100 divisions. An eyepiece graticule has 100 divisions that match the stage micrometer exactly. If a cell measures 5 eyepiece units, calculate the actual size of the cell in micrometres (\mu m).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Drawing cells from slides and photomicrographs
Examiners mark biological drawings against fixed rules:
🧑🏫 Teacher's explanation
Imagine you are an architect tasked with drawing a floor plan. If you are drawing a quick sketch for a friend, you might use messy, scribbled lines. But if you are drawing a professional blueprint, you use a sharp pencil, single continuous lines, and precise measurements. Biological drawings are like those blueprints; they are not 'artistic' impressions, but technical records of what is actually visible.
📖 Definitions
🔍 Worked example — Standard
A micrograph shows a mitochondrion that is 12 mum long. The image of the mitochondrion measures 36 mm. Calculate the magnification.
🔍 Worked example — Exam-style
An electron micrograph shows a section of chloroplast. The actual width of the chloroplast is 5 mum. The width of the image is 50 mm. Calculate the magnification.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the magnification of the image. [2] (b) Explain why an electron microscope is preferred over a light microscope for studying the internal structure of the bacterium. [2] [4 marks]
Worked example
Question. A photomicrograph of a cell shows a mitochondrion with a measured length of 45 mm on the image. The image is labelled with a magnification of ×15 000. Calculate the actual length of the mitochondrion in micrometres. [3]
🔍 Worked example — Easy
A student measures a plant cell under a light microscope. The image of the cell is 20 mm long. The magnification is × 400. Calculate the actual length of the cell in mum.
🔍 Worked example — Standard
An electron micrograph shows a bacterium that is 2 mum long. The image of the bacterium is 40 mm long. Calculate the magnification used.
🔍 Worked example — Exam-style
A researcher uses a stage micrometer to calibrate an eyepiece graticule. The stage micrometer scale shows that 10 divisions equal 0.1 mm. If the same 10 divisions on the eyepiece graticule correspond to 5 units on the eyepiece scale, calculate the value of one eyepiece graticule unit in mum.
⚠️ Common errors
Common mistakes
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